Explanation: The famous "Einstein Cross" is a
case where a single object is seen four times. Here a very distant
QSO happened to be placed right behind
a massive galaxy. The gravitational
effect of the galaxy on the distant QSO
was similar to the lens effect of an empty wine glass on a distant
street light - it created multiple images. But stars in the foreground
galaxy have been found to act as gravitational lenses
here too! These stars make the images change brightness relative
to each other. These brightness changes are visible on these two
photographs of the Einstein Cross,
taken about 3 years apart.